High-power transmitter valves dissipate large quantities of heat, which has to be removed from the environment in which they are contained in order not to prejudice their operation. The heat removal problem has been solved up to the present time by three different methods.
In a first method, the air is drawn from and expelled into the internal air-conditioned atmosphere of the station. The great drawback of this method is that the station air-conditioning system has to be dimensioned taking account of this increased thermal load, which requires the refrigeration power to be substantially increased. This method therefore requires overdimensioning both of the air conditioning system and of the uninterruptible power supply (UPS) if present in the station, besides representing a wastage of electrical energy.
In a second method, air is drawn in from the outside by way of filters and armoured electrical resistance heaters, which heat the intake air when necessary, the used air being directly expelled to the outside.
With regards to heating, it should be noted that high-power transmitter valves are cooled by air which enters at a temperature not less than a value prescribed by their manufacturer in order not to prejudice their operating characteristics. Moreover, if the system for switching-in the heater elements is not of continuous electronic type (which however is delicate and costly), but of the type comprising a thermostat and electromagnetic switch (and thus robust and economical), considerable transients in the inlet air temperature can arise on switching in the heater elements, which could result in instability.
In a third method, the air is drawn from and expelled into a separate environment of relatively small dimensions. The air temperature in this environment is controlled by a system comprising an electronic amplifier and motorised actuator which expels the excess hot air fraction directly to the outside. This third method is effective, but requires a certain enclosed space which can be problematical to find in prefabricated and/or transportable stations. The active control system also requires an electrical supply and cables, and is subject to faults as in the case of any electronic equipment.